1. Field of the Invention
This invention, in the field of immunology/immunotherapy, vaccine discovery and development, relates generally to the identification of immunogenic peptides from regions of proteins and molecules that are involved in the binding interactions with polyclonal and monoclonal antibodies and other specific binding peptides/molecules. The present invention is directed to methods for identification and use of the peptides for preventing, suppressing and treating immune-related diseases. Specifically, the invention provides therapy that result in clinical improvement in cancer patients.
2. Description of the Background
Autoimmune diseases are characterized by an unwanted and unwarranted attack by the immune system on the tissues of the host. While the mechanism for progress of these diseases is not well understood, at least some of the details with respect to antigen presentation are known. It is thought that antigens, including autoantigens, are processed by antigen-presenting cells (APC), and the resulting fragments are then associated with one of the cell surface proteins encoded by the major histocompatibility complex (MHC). As a result, recognition of a peptide antigen is said to be MHC “restricted.” When the MHC/antigen fragment complex binds to a complementary T cell receptor (TCR) on the surface of a T lymphocyte, activation and proliferation of the clone or subpopulation of T cells result bearing that particular TCR. Once activated, T cells have the capacity to regulate other cells of the immune system which display the processed antigen and to destroy the cells or tissues which carry epitopes of the recognized antigen.
Antibody therapies in which antibodies are directed to MHC molecules and CD4 molecules have been generally successful in several animal models of autoimmunity. However, these approaches may be too nonspecific and potentially overly suppressive. This may be because 70% of T cells bear the CD4 marker and because all T cell-mediated responses and most antibody responses require MHC-associated antigen presentation.
A major difficulty with present approaches is that they require the use of complex biological preparations which do not comprise well-defined therapeutic agents. Such preparations suffer from complex production and maintenance requirements (e.g., the need for sterility and large quantities of medium for producing large number of “vaccine” T cells), and lack reproducibility from batch to batch. To be useful in humans, T cell “vaccine” preparations must be both autologous and individually specific. This means they must be uniquely tailored for each patient. Furthermore, the presence of additional antigens on the surface of such T cells may result in a broader, possibly detrimental, immune response not limited to the desired T cell clones (Offner et al., J. Neuroimmunol. 21:13-22 (1989).
There is a need, therefore, for agents and pharmaceutical compositions which have the properties of specificity for the targeted immune response. These agents and compositions should also have predictability in their selection, convenience and reproducibility of preparation, and sufficient definition in order to permit precise control of dosage.
An effective vaccine is capable of generating a long-lasting immunity while being relatively harmless to the recipient. Attenuated organisms and purified antigens from organisms have traditionally been used as vaccines. However, such agents often produce deleterious side effects or fail to protect against subsequent challenges. Because of the inherent difficulties in growing pathogenic organisms and producing effective vaccines from them, many viral, bacterial and parasitic diseases have no effective vaccine.
A further difficulty with the use of peptides as vaccines is that, in most instances, peptides alone are not good immunogens. It is a well known phenomenon that most immune responses to peptide antigens are T cell-dependent. Accordingly, “carrier” molecules have been attached to peptide antigens that bind, for example, to B cell surface immunoglobulin in order to generate a high affinity, IgG response. In other words, nonresponsiveness to peptide antigens may sometimes be overcome by attaching another peptide that induces helper T cell activity.
In general, peptides that induce helper T cell activity are generated by B cells from enzymatic digestion of native proteins internalized by way of an antibody receptor. These T cell stimulating peptides are then presented on the surface of the B cell in association with class II major histocompatibility complex (MHC) molecules. In a similar fashion, peptides that induce cytotoxic T cell activity may be generated by accessory cells, including B cells. These peptides are presented on the cell surface of accessory cells in association with class I MHC molecules. As used herein, the term “T cell stimulatory peptide” means any peptide which activates or stimulates T cells, including (but not limited to) helper T cells and/or cytotoxic T cells.
Peptides represent a promising approach to the production and design of vaccines. However, the difficulties in making peptides that induce the desired immune response have hampered their success. This includes the difficulties inherent in making peptides that closely mimic the native structure of antigenic determinants.
These antigenic determinants, or epitopes, of a protein antigen represent the sites that are recognized as binding sites by certain immune components such as antibodies or immunocompetent cells. While epitopes are defined only in a functional sense, i.e. by their ability to bind to antibodies or immunocompetent cells, there is a structural basis for their immunological activity.
Epitopes are classified as either being continuous and discontinuous. Discontinuous epitopes are composed of sequences of amino acids throughout an antigen and rely on the tertiary structure or folding of the protein to bring the sequences together and form the epitope. In contrast, continuous epitopes are linear peptide fragments of the antigen that are able to bind to antibodies raised against the intact antigen.
Many antigens have been studied as possible serum markers for different types of cancer because the serum concentration of the specific antigen may be an indication of the cancer stage in an untreated person. As such, it would be advantageous to develop immunological reagents that react with the antigen. More specifically, it would be advantageous to develop immunological reagents that react with the epitopes of the protein antigen.
Conventional methods using biochemical and biophysical properties have attempted to determine the location of probable peptide epitopes. These methods include a careful screening of a protein's primary structure, searching for critical turns, helices, and even the folding of the protein in the tertiary structure. Continuous epitopes are structurally less complicated and therefore may be easier to locate. However, the ability to predict the location, length and potency of the site is limited.
Various other methods have been used to identify and predict the location of continuous epitopes in proteins by analyzing certain features of their primary structure. For example, parameters such as hydrophilicity, accessibility and mobility of short segments of polypeptide chains have been correlated with the location of epitopes.
Hydrophilicity has been used as the basis for determining protein epitopes by analyzing an amino acid sequence in order to find the point of greatest local hydrophilicity. As discussed in U.S. Pat. No. 4,554,101, each amino acid is assigned a relative hydrophilicity numerical value which is then averaged according to local hydrophilicity so that the locations of the highest local average hydrophilicity values represent the locations of the continuous epitopes. However, this method does not provide any information as to the optimal length of the continuous epitope. Similarly, U.S. Pat. No. 6,780,598 B1 determines the immunopotency of an epitope by providing a ranking system delineating between dominant and subdominant epitopes.
Computer-driven algorithms have been devised to take advantage of the biochemical properties of amino acids in a protein sequence by sorting information to search for T cell epitopes. These algorithms have been used to search the amino acid sequence of a given protein for characteristics known to be common to immunogenic peptides. They can often locate regions that are likely to induce cellular immune response in vitro. Computer-driven algorithms can identify regions of proteins that contain epitopes which are less variable among geographic isolates, or regions of each geographic isolate's more variable proteins, or perform as a preliminary tool to evaluate the evolution of immune response to an individual's own quasi species.
Peptides presented in conjunction with class I MHC molecules are derived from foreign or self protein antigens that have been synthesized in the cytoplasm. Peptides presented with class II MHC molecules are usually derived from exogenous protein antigens. Peptides binding to class I molecules are usually shorter (about 8-10 amino acid residues) than those that bind to class II molecules (8 to greater than 20 residues).
Identification of T cell epitopes within protein antigens has traditionally been accomplished using a variety of methods. These include the use of whole and fragmented native or recombinant antigenic protein, as well as the more commonly employed “overlapping peptide” method for the identification of T cell epitopes within protein antigens which involves the synthesis of overlapping peptides spanning the entire sequence of a given protein. Peptides are then tested for their capacity to stimulate T cell cytotoxic or proliferation responses in vitro.
The overlapping peptide method is both cost and labor intensive. For example, to perform an assay using 15 amino acid long peptides overlapping by 5 amino acids spanning a given antigen of length n (a small subset of the possible 15-mers spanning the protein), one would need to construct and assay (n/5)-1 peptides. For most types of analyses, this number would be prohibitive.
Accordingly, a simple method to identify immunogenic peptides from regions of self-proteins and other proteins and molecules involved in binding interactions with polyclonal and monoclonal antibodies is needed.